The invention relates generally to wireless communications between a base station and multiple users. More particularly, the invention relates to a system and method for dynamically optimizing a transmission mode of wirelessly transmitted information.
Wireless communication systems commonly include information carrying modulated carrier signals that are wirelessly transmitted from a transmission source to one or more receivers within an area or region.
Wireless communication systems serving stationary and mobile wireless subscribers are rapidly gaining popularity, resulting in a need for greater efficiency in the use of the available radio frequency spectrum. This goal is complicated because wireless communications channels between transmit and receive devices are inherently variable, so the characteristics of wireless channels, such as signal quality, generally vary in time, frequency and space. Under good conditions wireless channels exhibit good communication parameters, e.g., large data capacity, high signal quality, high spectral efficiency and throughput. However, under poor channel conditions, these parameters have significantly lower values. For example, when the wireless channel is degraded the transmitted data may experience excessive corruption, manifesting as high bit-error rates or packet error rates. The degradation of the channel can be due to a multitude of factors such as general noise in the channel, multipath fading, loss of line-of-sight path, excessive Co-Channel Interference (CCI) and other factors.
Motivated by these complications, prior art wireless systems have employed adaptive modulation of the transmitted signals with the use of feedback from the receiver as well as adaptive coding and receiver feedback to adjust data transmission to changing channel conditions. Such adaptive modulation has been applied to Single Input Single Output (SISO) as well as to Multiple Input Multiple Output (MIMO) systems, e.g., systems with antenna arrays at both the transmit and receive ends.
In wireless systems (mobile and fixed), signal degradation and corruption is primarily due to interference from other cellular users within or near a given cell and multipath fading, in which the received amplitude and phase of a signal varies over time. In Fixed Wireless Access (FWA) systems, that is, where the receiver remains stationary, signal fading rate is less than in mobile systems. In this case, the channel coherence time or the time during which the channel estimate remains stable is longer since the receiver does not move.
Prior art wireless systems have employed adaptive modulation of the transmitted signals with the use of feedback from the receiver as well as adaptive coding and receiver feedback to adapt data transmission to changing channel conditions. Such adaptive modulation is applied to Single Input Single Output (SISO) systems. In both SISO and MIMO systems, however, the fundamental problem of efficient choice of the mode to be applied to the transmitted data remains.
It would be an advance to provide a mode selection technique which allows the system to rapidly and efficiently select the appropriate mode for encoding data in a quickly changing channel. It is important that such technique be efficient in all wireless systems, including Multiple Input Multiple Output (MIMO), Multiple Input Single Output (MISO), Single Input Single Output (SISO) and Single Input Multiple Output (SIMO) systems as well as systems using multiple carrier frequencies, for example, OFDM systems.
The invention includes an apparatus and a method for adaptively optimizing a transmission mode of data transmitted to users within a wireless cellular system. The apparatus and method are adaptable for use in MIMO systems.
A first embodiment of the invention includes a method of optimizing a transmission mode of wirelessly transmitted data. The method includes selecting a first transmission mode based on a predetermined channel database and a first channel characterization. The first channel characterization can be based upon signal transmission according to an initial mode. An error factor is generated based on a difference between an estimated performance characteristic, and an expected performance characteristic. A subsequent transmission mode is selected based upon the predetermined channel database, the error factor and a subsequent channel characterization.
A second embodiment is similar to the first embodiment. For this embodiment, the predetermined channel database includes a predetermined look-up-table that provides a transmission mode selection based upon a channel characterization. The look-up-table generally includes a plurality of quality parameter thresholds that determine the selection of a transmission mode.
The second embodiment can further include adjusting the quality parameter thresholds within the predetermined look-up-table with the error factor, and selecting the subsequent transmission mode based upon the adjusted look-up-table and the subsequent channel characterization.
The error factor can be generated once for every received data packet. One embodiment includes the error factor being set to a first value if a data packet is properly received, and the error factor being set to a second value if the data packet is improperly received.
The error factor can also be generated once per a predetermined amount of time. The error factor can be set to a first value if an average packet error ration is greater than an upper bound threshold, and the error factor can be set to a second value if the average packet error ration is less than an lower bound threshold.
Another embodiment includes the quality parameter thresholds within the predetermined look-up-table being adjusted by the error factor after the reception of each data packet. Another embodiment includes adjusting the subsequent channel characterization with the error factor, and selecting the subsequent transmission mode based upon the look-up-table and the adjusted subsequent channel characterization.
A third embodiment is similar to the first embodiment. The third embodiment includes spatial multiplexing. The third embodiment includes individually selecting an first transmission mode based on a predetermined channel database and a first channel characterization for each of a plurality of transmission streams. Each of the plurality of transmission streams are received by the same receiver, allowing spatial multiplexing. An error factor is generated based on a difference between an estimated performance characteristic, and an expected performance characteristic, of the plurality of transmission streams. A subsequent transmission mode is selected based upon the predetermined channel database, the error factor and a subsequent channel characterization, for the transmission streams. Another embodiment includes the transmission streams being transmitted from a plurality of base stations, providing multiple base station spatial multiplexing. For multiple base station spatial multiplexing, an error factor and a corresponding subsequent transmission mode can be generated for each of the transmission streams.
A fourth embodiment includes method of optimizing a transmission mode of wirelessly transmitted data. The method includes receiving transmission signals that include data encoded in an initial transmission mode. A first quality parameter of the received transmission signals is measured. A subsequent transmission mode is selected based upon the quality parameter. Transmission signals are received having data encoded in the subsequent transmission mode. A second quality parameter is measured. A parameter is adjusted within selection criteria of another subsequent transmission mode based upon the second quality parameter.
Selecting a subsequent transmission mode based upon the quality parameter can include referencing a predetermined look-up-table that provides a subsequent transmission mode selection based upon the first quality parameter. The look-up-table can include a plurality of quality parameter thresholds that determine the selection of a subsequent transmission mode.
Adjusting a parameter within a selection criteria of the subsequent transmission mode based upon the second quality parameter can include adjusting the quality parameter thresholds within the predetermined look-up-table.
Obtaining the second quality parameter can include incrementing a table correction factor to a first value of a data packet is properly received, and incrementing the table correction factor to a second value if the data packet is improperly received. The quality parameter thresholds within the predetermined look-up-table are adjusted by the table correction factor after the reception of each data packet.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.